sfxge(4): rename firmware update verify result cap field

The existing name confuses support for secure boot with
support for reporting a verify result after an NVRAM update.

As the capability only reports support for returning a verify
result, change the name to be less confusing.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18088